This invention relates in general to imaging, and more specifically to a migration imaging system employing overcoated migration imaging members.
Recently, a migration imaging system capable of producing high quality images of high density, continuous tone, and high resolution has been developed. Such migration imaging systems are disclosed in copending applications Ser. No. 837,780, filed June 30, 1969, and Ser. No. 837,591, filed June 30, 1969. In a typical embodiment of the new migration imaging system an imaging member comprising a substrate, a layer of softenable material and photosensitive marking material is latently imaged by electrically charging the member and exposing the charged member to a pattern of activating electromagnetic radiation such as light. Where the photosensitive marking material was originally in the form of a fracturable layer contiguous the upper surface of the softenable layer, the marking particles in the exposed areas of the member migrate toward the substrate when the member is developed by softening the softenable layer.
"Softenable" as used herein is intended to mean any material which can be rendered more permeable thereby enabling particles to migrate through its bulk. Conventionally, changing the permeability of such material or reducing its resistance to migration of migration marking material is accomplished by dissolving, melting, and softening, by methods, for example, such as contacting with heat, vapors, partial solvents, solvent vapors, solvents and combinations thereof, or by otherwise reducing the viscosity of the softenable material by any suitable means.
"Fracturable" layer or material as used herein, means any layer or material which is capable of breaking up during development, thereby permitting portions of said layer to migrate toward the substrate or to be otherwise removed. The fracturable layer may be particulate, semi-continuous, or microscopically discontinuous in various embodiments of the migration imaging members of the present invention. Such fracturable layers of marking material are typically contiguous the surface of the softenable layer spaced apart from the substrate, and such fracturable layers may be near, at, coated onto, or slightly, partially or substantially embedded in the softenable layer in the various embodiments of the imaging members of the inventive system. "Contiguous" for the purpose of this invention is defined as in Webster's New Collegiate Dictionary, Second Edition, 1960: "In actual contact; touching; also, near, though not in contact; adjoining," and is intended to generically describe the relationship of the fracturable layer of marking material in the softenable layer, vis-a-vis the surface of the softenable layer spaced apart from the substrate.
There are various other systems for forming such images, wherein non-photosensitive or inert, marking materials are arranged in the aforementioned fracturable layers, or dispersed throughout the softenable layer, as described in the aforementioned copending applications which also disclose a variety of methods which may be used to form latent images upon such migration imaging members.
Various means for developing the latent images in the novel migration imaging system may be used. These development methods include solvent wash-away; solvent vapor softening, heat softening, and combinations of these methods, as well as any other method which changes the resistance of the softenable material to the migration of particulate marking material through said softenable layer to allow imagewise migration of the particles toward the substrate. In the solvent wash-away development method, the migration marking material migrates in imagewise configuration toward the substrate through the softenable layer as it is softened and dissolved, leaving an image of migrated particles corresponding to the desired image pattern on the substrate, with the material of the softenable layer substantially completely washed away. In the heat or vapor softening developing modes, the softenable layer is softened to allow imagewise migration of marking material toward the substrate and the developed image member generally comprises the substrate having migrated marking particles near the softenable layer-substrate interface, with the softenable layer and unmigrated marking materials intact on the substrate in substantially their original condition.
Various methods and materials and combinations thereof have previously been used to fix unfixed migration images. For example, fixing methods and materials previously used are disclosed in copending applications Ser. No. 590,959, filed Oct. 31, 1966 now abandoned, and Ser. No. 695,214, filed Jan. 2, 1968.
In addition to the aforementioned copending applications, another copending application Ser. No. 71,781 filed Sept. 14, 1970, discloses a migration imaging system which relates to transparentizing background portions of an imaged member, apparently by an agglomeration effect. In that system an imaging member comprising a softenable layer containing a fracturable layer of electrically photosensitive migration marking material is imaged in one process mode by electrostatically charging the member, exposing the member to an imagewise pattern of activating electromagnetic radiation, and then softening the softenable layer by exposure for a few seconds to a solvent vapor thereby causing a selective migration of the migration material in the softenable layer in the areas which were previously exposed to the activating radiation. The vapor developed member is then subjected to a heating step causing the migration material in unexposed areas to agglomerate or flocculate, often accompanied by fusion of the marking material particles, thereby resulting in a very low background image. Alternatively, the migration image may be formed by heat followed by exposure to solvent vapors and a second heating step which results in background reduction. In this imaging system, the softenable layer remains substantially intact after development, with the image being self-fixed because the marking material particles are trapped within the softenable layer. In the embodiments thereof the final migration image having low background is typically formed by some combination of vapor-heat treatment.
In new and growing areas of technology such as the migration imaging systems of the present invention, new methods, apparatus, compositions of matter, and articles of manufacture continue to be discovered for the application of the new technology in new modes. The present invention relates to a new and advantageous migration imaging system employing overcoated imaging members.